Patent GB-1266521-A (GB)
IPC: C03B37/027
Filed: 1969-03-14
Abstract: 1,266,521. Fibre optics. NIPPON SELFOC K.K. March 14, 1969 [March 15, 1968; Nov. 18, 1968(2)], No. 13600/69. Heading G2J. [Also in Division Cl] Glass fibres or fibre structures are formed from glass comprising at least two thirds of cations of modifying oxides (not themselves the operative glass forming oxide) the concentrations of which vary progressively, one increasing and one decreasing, from the centre axis towards the side surface to create a refractive index distribution which increases or decreases from the centre axis towards the side surface. The path of light 21 through a fibre 22 is then as shown in Fig. 2. Examples of modifying oxides are Li 2 O, Na 2 O, K 2 O, Rb 2 O, CS 2 O, Tl 2 O, An 2 O, Ag 2 O, Cu 2 O, BeO, MgO, CaO, BaO, SrO, ZnO, CdO, PbO, SnO 2  and La 2 O 3 , and for example, for silicate glasses, the order of contribution to increase of refractive index is given as Pb, Ba, Cd, Sr, Ca, Zn, Be, Mg, and Tl, Li, K, Na, Rb, Cs (the last four similar) respectively for divalent and monovalent oxides. The following are given as examples for the different pairs of modifying oxides:- (1) Thallium ion - at least one alkali metal ion. (2) Alkaline earth metal ion with larger atomic number - alkaline earth metal ion with smaller atomic number. (3) Lead ion - at least one alkaline earth metal ion. (4) Lead and/or Barium ion - K, Na and/or Li ion. (5) Pb, Cd, Zn, Ba, Sr and/or Ca ion - at least one alkali metal ion. Glasses suitable as the basic glass are silicate, borate and phosphate glasses. The prepared refractive index distribution is such that it can be represented by where: - No is the refractive index of the glass at the central part of the fibre r is the radial distance from the fibre centre N is the refractive index of the glass at distance r, and a is a positive constant.  Many examples of compositions are given and the variation of a is from 0À92 to 90. Manufacture. The glass fibres are produced from rods or fibres of a first glass containing one of the modifying oxides which are contacted by a source of the second modifying oxide at a temperature such that ion substitution can take place, the source of the second oxide being either a bath of a salt containing the oxide in solid, liquid or vapour form or a second glass fused to the first glass. A further glass may be used as an intermediate glass. When rods are initially used, they are then drawn out into fibres. Examples of baths and glasses are given. Apparatus. Single glass fibres may be drawn from a glass rod surrounded by a glass tube and drawn downwardly or from two or more molten glasses, one container surrounding the other which may surround another. To produce a complex glass fibre structure, as in Fig.15 (not shown), with fibres 151a to 151g of one glass surrounded by a second glass 152, double concentric pots 161, 163, Fig. 16 (not shown), of platinum have constructed nozzles, the inner pot 161 having a number of nozzle tips 162 are used, the pots being heated electrically and the resultant fibre being drawn down. A plurality of glass fibres may be secured together and drawn downwardly. The glass fibres clad with a further glass utilize glasses having matching coefficients of thermal expansion, the ion exchange taking place automatically during cooling from the hot glass. In a further method utilizing a bath, a hollow tube of glass is filled with the ion-exchange salt and heated for the required time to effect ion-exchange, the tube then being emptied and rinsed before being reheated and drawn in a vacuum to give a solid glass fibre.